Cercospora leaf blight, ASR remain threats to Louisiana soybeans

The 2006 was one of the hottest years Boyd Padgett can remember since moving back to his home state a decade ago. And the LSU AgCenter plant pathologist, stationed in northeast Louisiana, expects those hot conditions to “directly affect” the 2007 growing season.

“We saw very little disease, state-wide,” Padgett said at the recent Louisiana Soybean Association annual meeting at the Dean Lee Research Station in Alexandria, La. “That means we may have a slow start for diseases this year.”

Plant pathologists “are wondering if the Asian soybean rust (ASR) pathogen will overwinter in Louisiana kudzu. We're checking kudzu patches in St. Landry Parish and also along the coast.”

The fears are stoked by continuing ASR discoveries in the Southeast.

“We already have reports of ASR showing up in Alabama, Florida and Georgia. It seems we're beginning to see this disease earlier each year. Last year, on Jan. 16, ASR had shown up in one little spot. The difference between then and now is striking.”

Soybean diseases

As for other diseases of concern in Louisiana soybeans, cercospora leaf blight leads the list.

“It's the worst disease problem we have to deal with year in and year out. It's all over the state — and gets worse the farther south you go.

“We also have aerial blight. That disease can be every bit as devastating as ASR. That should be kept in mind, particularly in the southern parishes.”

Frogeye leafspot isn't a problem every year — it's sporadic. But when it hits, it can cut soybean yields.

As for pod and stem diseases, Padgett sees three every year: anthracnose, purple seed stain and pod and stem blight.

One disease that continues to show up in Louisiana's soybeans is stem canker.

“I'm getting more and more reports of it. I don't think there have been any cases of fields being wiped out by it but I'm seeing much more of it.”

Red crown rot, a soil-borne organism, is another disease that is showing up. “It can be managed to some extent by planting when it's favorable for seed germination.”

What does it take to develop an effective disease management program? One thing is “to know what problems are associated with your field. Also, there is genetic resistance to some disease-causing organisms and growers need to capitalize on that. Pick varieties for that resistance.”

One thing that's encouraging to Padgett is researchers are finding varieties that slow the rust pathogen's advance. “I wouldn't call it resistance, but that would complement our fungicides.”

What about variety evaluations?

“I'd encourage you to choose varieties that have been tested near where you're growing a crop. You may say, ‘Well, why should I do that. I'll just check the averages across all locations.’”

Padgett offered an example of why that's a bad idea. In research work done in Jefferson Davis Parish, “there are four varieties that appear to have some resistance to cercospora blight. But look at the same varieties in northeast Louisiana in a variety test and all were affected by the disease organism.”

What's going on?

“Well, several things. First, the environment can be different. Ray Schneider, a LSU AgCenter plant pathologist working in Baton Rouge, has shown a vast diversity within the pathogen population. That means the cercospora in south Louisiana may be a little different than the one in the north. So, choose varieties based on studies done in your area of the state.”

Seed treatments

What about seed treatments?

“There's a big interest in seed treatments in many crops. I've tested a number of compounds over the last couple of years.

Among Padgett's fungicide observations:

In tests conducted in northeast Louisiana, the treatments do seem to allow the seed to jump out of the ground a bit quicker. “However, I've seen no effect on the final stand.”

Seed treatments have not affected plant height. “In fact, some of the experimental compounds I've checked have actually stunted plants.”

Padgett has “never seen a yield boost from a seed treatment.”

However, “I do think seed treatments are useful. In situations where you have to plant early, or if you're using a variety where the seed quality is marginal or are planting into poorly-drained or some no-till situations, seed treatments may benefit you.”

Don't forget proper foliar application techniques.

“You can take a good fungicide that works well against a disease organism. But if you don't apply it properly, you'll have wasted money.”

Coverage is vital. “If you put out a fungicide by air, we'd like to see it go out at 4 gallons per spray volume per acre, preferably 5 gallons.”

When working a ground application, “put out 10 to 20 gallons per acre.”

Back in the 1970s and 1980s there were few soybean fungicide options.

“Really, we had only Benlate or Bravo. Now, we have a multitude to choose from. Quadris and Headline are probably the backbone of our fungicide program. Topsin-M is also very good and popular.”

There isn't a fungicide that completely “takes out cercospora blight. If we had one, it would be our number one fungicide.”

There are also some pre-mixes available.

“Headline SBR, Quilt and Stratego are on the shelf. Those contain products active against ASR and other diseases. Basically, we have to spike them with Headline or Quadris or a strobilurin in order to control, or at least suppress, cercospora blight.”

Other fungicides that will primarily be used to control ASR include Laredo, Domark, and Folicur.

“Those are triazoles and provide protection. They're systemic and are supposed to be preventive and provide kick-back activity. Tilt and Propamax are also triazoles that are okay against ASR.”

Among the newer fungicides coming down the pipe is Absolute, “a premix of Folicur and another product like Quilt or Headline SBR. Alto, Caramba, Charisma, and Quadris Extra may also be available in 2007.”

Padgett recently did a study evaluating the timing of fungicide applications. As far as cercospora blight in 2006, “we found little difference between Quadris, Topsin-M, or Headline applied at R-1, R-3 or R-5. However, disease pressure was low and the environment was hot and dry.

“In 2005, we found the R-1 application was too early. I don't think that early application will be feasible for us unless ASR is looming. It appears that applications made between R-3 and R-5 are our best bet.”